This invention relates to load indicating fasteners that are “thread-forming” (also referred to as “thread-rolling” or “self-tapping” fasteners), methods for making load indicating thread-forming fasteners, and methods for measuring the load in thread-forming fasteners.
Thread-forming fasteners are well known in many industries, such as in high-volume automotive assembly. Examples of such fasteners are described in U.S. Pat. No. 5,242,253 (Fulmer), issued Sep. 7, 1993, for example. Such fasteners are also marketed commercially, for example, by Reminc, Research Engineering and Manufacturing Inc., Middletown, RI, USA, under the trademark “Taptite” and “Taptite 2000”, and a description of such fasteners can be found in their product literature, entitled “Taptite 2000 Thread Rolling Fasteners”.
The major advantage of thread-forming fasteners is that they can be installed directly into a drilled hole, eliminating the cost of tapping the hole. Additionally, the thread formed by a thread-forming fastener has very tight tolerance since it is formed by the fastener itself and therefore forms a better nut.
Although thread-forming fasteners have been used in numerous applications in the automotive and aerospace industries to reduce cost, such fasteners are generally restricted to non-critical or less-critical applications. The difficulty in controlling the tightening process prevents their use in critical applications.
The primary reason for this is that the thread-forming process requires torque, in addition to the tightening torque, and this thread-forming torque varies significantly with hole tolerance, material, friction conditions, etc. As a result, the precise tightening of a thread-forming fastener to a specified torque into a blind hole, where the thread is still being formed as the bolt is being tightened, will result in a 3 sigma load scatter of typically +/−50%, which is unacceptable in critical applications.